This invention relates to an optical sheet for enhancing the directivity of light and a backlight and a liquid crystal display apparatus which include the optical sheet.
In recent years, to a liquid crystal display apparatus which includes a liquid crystal panel, it is a significant subject to enhance the display luminance together with reduction of the power consumption in order to raise the commodity value of the liquid crystal display apparatus. In such a situation as just described, it is demanded strongly to improve the optical gain on the backlight side. Thus, it has been proposed, as a method which satisfies the demand, to provide a liquid crystal display apparatus with a prism sheet which includes a prism array on the emitting side of illumination light (refer to, for example, Japanese Patent No. 3147205).
FIG. 1 shows an external appearance of a conventional prism sheet. FIG. 2 shows a shape of an XZ section of the conventional prism sheet. The prism sheet can separate an incident light ray into a first-order transmission light component T1 which is transmitted directly through a prism lateral face, a return light component R which is reflected by the prism lateral face and then reflected by the other prism lateral face so that it is returned to the incidence side, and a second-order transmission light component T2 which is reflected by the prism lateral face and then transmitted through the other prism lateral face so that it is emitted to a front face of the prism sheet depending upon the incident angle.
The first-order transmission light component T1 is a light flux component which includes light emitted in a front face direction and is utilized effectively. The return light component R is a light flux component which enters and is diffused and reflected by the diffusion sheet, which is regarded as a light emitting face (planar light source), and is effective to increase the luminance of the light emitting face. The second-order transmission light component T2 is a light flux component which is emitted to the wide angle side outside an effective angular field of view of the liquid crystal panel and does not contribute to enhancement of the luminance.
In this manner, the conventional prism sheet refracts and transmits incident light therethrough to condense the incident light in the front face direction thereby to improve the directivity characteristic so as to increase the front face luminance. Further, since reflected light is diffused and scattered by the diffusing sheet which is regarded as a light emitting face (planar light source) to increase the luminance of the light emitting face, the front face luminance increases.
As described above, the conventional prism sheet can separate an incidence light ray into one of the first-order transmission light component T1, second-order transmission light component T2 and return light component R depending upon the incidence angle of the incidence light.
In the conventional prism sheet, as seen in FIG. 2, part of a light flux emitted from an off-axis imaginary light source is totally reflected by one of lateral faces of the prism sheet and re-enters the other lateral face, whereafter it advances in the inside of the sheet and is re-utilized as the return light component R. Or, the part of the light flux is effectively utilized, after multiple reflections thereof, as the first-order transmission light component T1 or as the return light component R to the light source side.
However, some light flux emitted from an off-axis imaginary light source is totally reflected by one of lateral faces of the prism sheet and then is refracted by and transmitted through the other lateral face so that it makes the second-order transmission light component T2 which is emitted to the wide angle side outside the effective angular field of view of the liquid crystal panel. The second-order transmission light component T2 is a light flux component which is ineffective to improvement of the luminance.
Further, depending upon the angle dependent characteristic of a polarized light separation sheet disposed on the succeeding stage, extreme deterioration of the polarized light separation characteristic is sometimes caused by the directivity of incident light, which makes an obstacle to effective luminance enhancement to the liquid crystal panel side.
Further, where the prism sheet described above is interposed between a diffuser and a liquid crystal panel, external appearance blurring appears. Therefore, it is demanded to suppress occurrence of external appearance blurring.